internal build up question

I’m planning to order a garden room from Kingspan insulated panel store. The rooms come with just the wall panels which are metal both internally and externally. I want to do a nicer finish, so was thinking this:

100mm insulated panel
77mm battens (to allow for services)
18mm ply
25mm wedi board
3mm skim

the ply is so i can hang things off the wall & the wedi board is to prevent damp or mould.

For full clarity, the shed i want is 22sqm internally and it needs to be 20sqm or less in order to be exempt from building control… so i’m trying to come up with a build up that will get the internal floor area that little bit smaller. But i also don’t want to do anything that’s going to cause issues for my garden room. It’s going to be used as a ‘library room’ where i can store my books and hang out to read. Any suggestions welcome. Thanks!